1. Field of the Invention
This invention relates generally to access devices and more specifically to methods and apparatus for inhibiting further cutting of tissue upon penetration of a tissue wall.
2. Discussion of the Prior Art
Devices and procedures for providing an enlarged tubular access into a body cavity or body conduit, were first conceived when catheters became widely used for minimally invasive surgery. A catheter, which may have a diameter such as 5 French, is typically very flexible and therefore does not have the column strength necessary to puncture the skin or a vessel in order to accommodate insertion of the catheter. A method which is still the preferred procedure was developed whereby a common surgical needle is inserted through the skin and into the vessel. This needle is closely overlaid with a thin sheath or introducer, which is carried by the needle into the vessel. When the needle is removed, the introducer is left in place and provides the tubular access through which the catheter can then be inserted.
In more recent times, minimally invasive surgery has further advanced so that large body cavities such as the abdomen can be accessed through tubular devices, and the surgical procedures performed with long narrow instruments through those access devices. It is not surprising that these devices, commonly referred to as trocars, are introduced through the abdominal wall or other tissue barrier, in much the same manner as that employed by the catheter introducer systems. Thus, trocars typically include a puncturing device, commonly referred to as an obturator, and a closely spaced outer sheath or cannula. In this case the obturator may have an outside diameter such as 10 millimeters, where the cannula has a similar inside diameter. Once the cannula is in place, narrow surgical apparatus can be inserted through the cannula to perform common functions such as cutting, irrigating, aspirating, grinding, traction and removal of body parts.
While the above mentioned procedure for introducing catheters has remained satisfactory, this same procedure applied to trocars has not been effective for two primary reasons. First, the size of the required puncture is much larger than that associated with catheters. Second, the abdominal wall consists of a material having a much greater density than skin or vessel walls. The puncture required for a trocar must typically be made through muscle which provides a much higher resistance to entry. As a result of these two differences, forces as great as 40 pounds may be required to insert a trocar into a body cavity.
In order to accommodate forces of this magnitude, obturators have been formed from solid metal rods and provided with very sharp points and long cutting edges leading from the point to the outer circumference of the obturator. While this has had some effect on the force required for insertion, it has only aggravated the problem associated with the presence of organs in close proximity to the abdominal wall.
In order to avoid puncturing of these interior organs, it has been necessary to stop the forward movement of the trocar immediately upon penetration. Thus the procedure has called for a tremendous force in order to penetrate the abdominal muscle and an immediate stopping of that force at the point where there is no further resistance to forward movement. In some cases, physicians have attempted to avoid the significant forward pressure by twisting and turning the trocar. This has tended to significantly traumatize the incision.
More recently, attempts have been made to mechanically cover the sharp cutting tip and edges immediately following penetration. U.S. Pat. No. 4,654,030 discloses a sheath which is biased to move forwardly over the point of the trocar as soon as it penetrates the abdominal wall. Elaborate apparatus for biasing this sheath to the forward position have been complicated by requirements for a long throwing distance and a short throwing time.
The need has remained for an apparatus and method which can easily puncture (with a low force and providing a high degree of control) along a precise incision (providing low trauma and excellent healing characteristics) while avoiding any further cutting immediately following penetration.